Packaging machines for wrapping and sealing plastic film about an article conventionally utilize a heated wire to seal film layers to one another and to melt through the layers in order to separate one article from another as the articles pass through the machine. Current is supplied to the wire to heat the wire to a high temperature in order to effect the seal and cutting operation. The appearance of the resulting seal is fine and neat as the film shrinks tightly around the package, especially where polypropylene films are involved. Such hot wires are typically used to form both end seals and side seals.
As the wire contacts the film and performs its intended function, it loses heat to the film as well as to the surrounding environment. Accordingly, current must be continually or continuously supplied to the wire in order to maintain the appropriate wire temperature.
Typically the wire is a resistive element approximately 45-50 thousandths of an inch in diameter, therefore making it susceptible to temperature build-up, fatigue and failure. Thus, if the current to the wire is not properly controlled and the wire temperature becomes too high, the wire tends to break. For example, as machine speed increases, the current impulse sent to the seal wire to heat the wire to the appropriate temperature becomes more and more frequent, until such point that the seal system is, in effect, on at all times. The wire becomes more susceptible to failure as the film being sealed is not drawing away the excess heat (acting as a heat sink) as fast as the heat is being applied to the wire. The wire eventually softens, stretches, and breaks. This is a common occurrence particularly when proper operator attention is absent. Changing the wire requires that the machine be shut down, resulting in considerable loss of productivity.
U.S. Pat. No. 5,597,499 addresses this problem by providing a seal wire control system that controls the duration of heat impulses applied to the sealing wire. It utilizes an open loop configuration that regulates the heat applied to the seal wire based on the number of articles and the frequency that the articles are run through the wrapper. However, the versatility of this solution is limited.
It would be desirable to provide a seal system that is a closed loop feedback configuration that detects when the wire is too hot, and responds by intercepting the input of the heat in order to protect the wire from fatigue and failure.
These and other objects will be made apparent by reference to the following description and drawings.
The problems of the prior art have been overcome by the present invention, which provides a control system and apparatus for controlling current input to an electrical resistance element such as a seal wire. The system and apparatus of the present invention is a closed loop feedback modification to conventional systems, and takes advantage of the inherent expansion of the seal wire as it is heated. When the seal wire expands to a predetermined length, a at least a portion of the power supplied to the wire for heating is interrupted. The interruption will continue until the wire contracts to a length less than the predetermined length, whereupon the interruption will cease and normal operation is resumed for heating of the wire.
The device of the present invention thus directly monitors the expansion of the seal wire, and reduces or eliminates current applied to the seal wire when the expansion reaches a predetermined length. Current can then be increased, and preferably restored to its original level, when the seal wire contracts sufficiently to minimize the danger of breakage.
In a preferred embodiment, heat input to the seal wire is in two modes. The first mode is a constant heat mode, whereby a constant amount of current is passed through the wire at rest or in idle mode to keep the wire warm and ready for service in short notice. The second mode is an impulse heat mode, in which a specific duration pulse of current through the seal wire is applied as the seal wire is about to engage the film. The duration of the impulse mode is generally responsive to the rapidity of the sealing cycle (e.g., the sealing jaw operation). The second or impulse mode is the mode interrupted when the wire expands to a predetermined length in the preferred embodiment of the present invention.